1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to an electrostatic recording type image forming apparatus and an electrophotographic image forming apparatus, such as a copying machine or a laser beam printer, configured to develop an electrostatic image formed on an image bearing member with a developer, which is a mixture of toner and carrier.
2. Description of the Related Art
A conventional electrophotographic image forming apparatus, such as a copying machine, a printer, or a facsimile machine, includes the following components to perform respective operations:
an electrophotographic photosensitive member, which is a rotational drum-like image bearing member;
a charging device configured to evenly charge the photosensitive member to a predetermined polarity and potential;
an exposure device, which is an information writing unit, configured to form an electrostatic latent image on the charged photosensitive member;
a development device configured to visualize, as a developer image (toner image), the electrostatic latent image formed on the photosensitive member, using a developer (toner);
a transfer device configured to transfer the toner image from a surface of the photosensitive member onto a recording material, such as paper;
a cleaning device configured to clean the surface of the photosensitive member by removing a small amount of toner remaining on the surface of the photosensitive member after the transfer (residual developer or residual transfer toner); and
a fixing device configured to fix the toner image to the recording material.
The toner remaining on the photosensitive member after the transfer process is removed from the surface of the photosensitive member by the cleaning device. Then, the removed toner is recovered into the cleaning device as waste toner. In view of environmental protection and effective utilization of resources, the market has desired that image forming apparatuses generate no waste toner.
In this regard, a conventional image forming apparatus is configured to return residual transfer toner (waste toner) recovered in a cleaning device to a development device to recycle the waste toner.
Japanese Patent Application Laid-Open No. 2004-117960 (corresponding to U.S. Pat. No. 6,952,546) discusses a cleanerless type image forming apparatus that can remove residual transfer toner remaining on a photosensitive member after transfer from the photosensitive member with a development device by “development-concurrent cleaning” to recover and recycle the toner.
The development-concurrent cleaning is a processing operation for recovering residual transfer toner remaining on a photosensitive member after transfer into a development device at the time of a next development operation. A photosensitive member having residual transfer toner thereon is then charged and exposed to form an electrostatic latent image. Then, in the development-concurrent cleaning, during processing for developing an electrostatic latent image, residual transfer toner existing on a non-image portion, of the residual transfer toner remaining on the surface of the photosensitive member, is removed and recovered into the development device with a fog removing bias.
The “fog removing bias” is a fog removing potential difference (Vback), which is a potential difference between a direct-current voltage applied to a development device and a surface potential of a photosensitive member.
With the conventional method discussed in Japanese Patent Application Laid-Open No. 2004-117960, residual transfer toner is recovered to a development device to be recycled for developing an electrostatic latent image in a next operation. Thus, waste toner can be reduced or suppressed, and an operator's trouble in maintenance of an image forming apparatus can be saved.
In addition, since the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2004-117960 is a cleanerless type apparatus, the surface of a photosensitive member is not worn or torn by a cleaner. Thus, the coating thickness of the surface of a photosensitive member can be maintained sufficiently thick. Accordingly, the life of a photosensitive member can be prolonged. The cleanerless configuration is also useful in downsizing the entire image forming apparatus.
In a case where a cleanerless type image forming apparatus employing the above-described development-concurrent cleaning uses a contact charging device, which is a charging device that contacts a photosensitive member to charge the surface of a photosensitive member, the following phenomenon can occur.
In this regard, a toner whose charge polarity is reversed against a normal polarity, of residual transfer toner, may adhere to the contact charging device when the residual transfer toner remaining on a photosensitive member passes through a contact nip portion (charging portion) between the photosensitive member and the contact charging deice. This phenomenon may cause an excessive toner contamination on the contact charging device, which may further cause a charging failure.
More specifically, a toner (developer) includes a very small amount of toner whose polarity is essentially reversed against a normal polarity. In addition, a normal polarity toner may be reversed into an antipolarity toner. Furthermore, a charge-eliminated toner may have a small amount of charge.
That is, a residual transfer toner may include a normal polarity toner, an antipolarity toner, and a toner having a small amount of charge. Among these toners, an antipolarity toner or a toner having a small amount of charge may adhere to a contact charging device when passing through a contact nip portion between a photosensitive member and the contact charging device.
In order to remove and recover residual transfer toner remaining on a photosensitive member by the above-described development-concurrent cleaning, it is required that the residual transfer toner remaining on the photosensitive member to be recovered to a development device has a normal polarity and an amount of charge large enough to develop an electrostatic latent image with the development device.
An antipolarity toner and a toner having a small amount of charge cannot be removed from a photosensitive member or recovered to a development device, which may cause an image failure.
In order to prevent adhesion of a toner on a contact charging device, the following operation is required. That is, it is necessary to charge residual transfer toner remaining on a photosensitive member, in which a normal polarity toner, an antipolarity toner, and a toner having a small amount of charge mixedly exist, into a normal polarity. Furthermore, it is necessary to make the toner have a uniform amount of charge.
In this regard, a conventional image forming apparatus has two different auxiliary charging means, namely, a toner charge amount control means and a residual transfer toner uniformizing means (residual toner uniformizing means). The toner charge amount control means is disposed upstream of a contact charging device and downstream of a transfer means as viewed in the direction of rotation of a photosensitive member, and charges residual transfer toner. The residual toner uniformizing means is disposed upstream of the toner charge amount control means and downstream of the transfer means, and uniformizes residual transfer toner remaining on the photosensitive member.
Japanese Patent Application Laid-Open No. 2001-215798 (corresponding to U.S. Pat. No. 6,421,512) and Japanese Patent Application Laid-Open No. 2001-215799 each discuss a method for addressing the above-described problem by applying a constant level of direct current voltage to the residual toner uniformizing means and the toner charge amount control means.
In the conventional method discussed in Japanese Patent Application Laid-Open No. 2001-215798 and Japanese Patent Application Laid-Open No. 2001-215799, the residual toner uniformizing means uniformizes residual toner remaining on the photosensitive member after transfer, and then the toner charge amount control means charges the uniformized residual transfer toner on the photosensitive member into a normal polarity.
Subsequently, the contact charging device charges the surface of the photosensitive member. Then, the contact charging device charges the residual transfer toner, which has been charged by the toner charge amount control means, to an amount of charge large enough to be removed and recovered with the development device by the development-concurrent cleaning. Then, the charged toner is recovered by the development device.
Recent conventional image forming apparatuses employ a two-component development method, which uses a developer made of a mixture of non-magnetic toner and magnetic carrier. With the two-component development method, an image quality can be more stabilized and the life of an image forming apparatus can be longer than those in the case of using another development method that has been discussed so far.
In the case of using an image forming apparatus having a long life, a developer can be deteriorated after a long use. In this case, a carrier in the toner (developer) may be deteriorated. Thus, it is necessary for an operator of an image forming apparatus to exchange the developer after a long use of the image forming apparatus. In order to reduce a trouble of exchanging a developer, the following conventional image forming apparatuses and methods have been developed and marketed so far.
Japanese Patent Application Laid-Open No. 06-324565 discusses an image forming apparatus whose development devices for colors of black (Bk), yellow (Y), magenta (M), and cyan (C) respectively replenish a developer including a toner in an amount equivalent to the current consumption amount due to the development operation. If too much amount of developer may exist in the development device, the exceeding developer is discharged into a waste developer container, which is disposed externally from the development device.
The excessive developer is discharged at substantially the same time as a replenishment of the developer. Thus, an image forming apparatus can be downsized, the cost of manufacture can be reduced, and characteristics of the entire developer can be stabilized.
Furthermore, with the conventional method described above, an operator of an image forming apparatus does not need to exchange a developer or a development device. Accordingly, an operator can more easily perform maintenance of an image forming apparatus, and the running cost for the image forming apparatus can thus be reduced. Such a method is generally referred to as an “automatic developer exchange system”.
Prolonging the life of a photosensitive member and a development device and reduction or suppression of waste toner can be achieved by using the above-described cleanerless type image forming apparatus employing an “automatic developer exchange system”. Furthermore, with such an apparatus, frequency of exchange of photosensitive members, a development device, a developer, and a waste toner container can be lowered. Thus, the running cost for the image forming apparatus can be reduced, and maintenance operations by an operator of such an image forming apparatus can be performed at a relatively prolonged time interval.
In recent years, the market has desired a full color image forming apparatus that can operate at a high speed. In order to address market needs, an image forming apparatus employing a tandem image forming method has been developed and marketed.
The full color image forming apparatus employing the tandem image forming method includes a photosensitive member, a charging device, an exposure device, and a development device for each color of yellow, magenta, cyan, and black. With such a configuration, the full color image forming apparatus employing the tandem image forming method can form an image for each unit arranged in tandem. According to the tandem image forming method, images of respective four colors can be individually and simultaneously formed. Thus, an image can be output at a high speed.
However, the above-described cleanerless image forming method has the following problems. In a cleanerless image forming apparatus, a cleaner blade is not provided. Thus, an external additive of toner may adhere to the photosensitive member to be recovered to the development device. The external additive recovered to the development device can be accumulated in the development device and can affect and deteriorate carrier in the development device.
In a cleanerless type full color image forming apparatus employing the above-described tandem image forming method, an external additive transferred via a transfer medium from an upstream image forming unit may be retransferred to a downstream image forming unit. Thus, the retransferred external additive may be recovered into the development device.
As a result, a larger amount of external additive may be accumulated in the development device in a downstream unit than in an upstream unit, which may cause more considerable deterioration of carrier in the development device in a downstream unit than in an upstream unit.
Once carrier deterioration occurs, toner charging performance of carrier may degrade. Thus, various phenomena, such as toner fogging, toner scattering, uneven toner, or low density, may occur.
In addition, in the case of a cleanerless image forming apparatus, a retransferred toner, which is a part of a toner image formed by an image forming unit disposed on a downstream side in the direction of conveyance of a transfer medium, can be recovered to the development device, in addition to the residual transfer toner remaining in each image forming unit.
An image forming unit disposed on a more downstream side in the direction of conveyance of a transfer medium is provided with a larger number of upstream image forming units disposed upstream thereof. Accordingly, the amount of the retransferred toner is greater in a more downstream image forming unit.
A residual transfer toner and a retransferred toner are toners that have not been transferred onto a transfer medium even when a transfer field is applied by a transfer means of an image forming unit. Accordingly, most residual transfer toners and retransferred toners have a polarity reverse to a normal polarity or have no polarity.
When the above-described residual transfer toner or retransferred toner is recovered to the development device, the charging capacity of the development device degrades. In this case, image failure may occur. An amount of retransferred toner accumulated in a development device is larger in a development device of a downstream image forming unit than that in a development device of an upstream image forming unit. Accordingly, an image failure due to accumulation of retransferred toner can more easily occur in a development device of a downstream image forming unit than in a development device of an upstream image forming unit.
In order to address this phenomenon, it is useful to raise the amount of charge of a residual transfer toner or a retransferred toner to an appropriate level using an auxiliary charging member disposed on a downstream side of a transfer means in the direction of rotation of a photosensitive member and then recover to a development device the residual transfer toner or retransferred toner having an appropriate level of amount of charge.
However, since the amount of retransferred toner is larger in a downstream image forming unit than in an upstream image forming unit, it is more likely with respect to a downstream image forming unit that the auxiliary charging member is contaminated with an accumulated toner or external additive. Accordingly, it is more difficult to control the amount of charge of a residual transfer toner or a retransferred toner with respect to a downstream image forming unit than to an upstream image forming unit.
Thus, the amount of toner in a developer in a development device, whose charge polarity is reversed against a normal polarity, can easily increase in this case. Accordingly, various phenomena, such as toner fogging, toner scattering, uneven toner, or low density, may occur.
In particular, at a time close to an end of the life of an image forming unit, in the case where the charging performance of a carrier has degraded, such phenomena can easily occur. It is very difficult to completely suppress such a retransferred toner and a retransferred external additive.
In order to address this phenomena, the life of a developer can be prolonged by using a cleanerless image forming apparatus employing an automatic developer exchanging method. However, in the case of using the automatic developer exchanging method, since a developer to be replenished includes a carrier, the following problem may arise if a ratio of carrier included in a replenishing developer is raised too high.
In this case, the cost of manufacture of a replenishing developer becomes high. Furthermore, the amount of toner to be contained in a replenishing toner bottle may decrease due to an increase in a volume of the replenishing developer by a volume equivalent to the volume of the carrier. Moreover, because of the low toner ratio in the replenishing developer, the replenishing of the developer cannot be performed at an appropriate timing in the case of forming a high density image.